Composite article having a surface prepared for diffusion bonding

ABSTRACT

A composite article is manufactured by preparing a machined surface of the composite article for diffusion bonding. The method includes removing most of the exposed fibers by shot peening followed by acid etching. The acid etch may be followed by grit blasting and then another acid etch. After the method is applied to the surface, the article can be diffusion bonded to other composite article to form complex shapes.

This is a division of application Ser. No. 07/978,574 filed Nov. 19,1992.

TECHNICAL FIELD

The present invention relates to fiber-reinforced metal matrix compositearticles comprised of a ceramic fiber as the reinforcing material and atitanium alloy as the matrix material, and in particular, to a methodfor preparing a surface of such a composite article having exposedfibers so that the article can be diffusion bonded to another titaniumalloy article.

BACKGROUND OF THE INVENTION

Metal matrix composite articles combine the high strength and lightweight of ceramic reinforcement fibers with the attractive properties ofthe titanium alloy matrix in which the fibers are embedded. Currentmethods for embedding the fibers within the matrix limit the final shapeof the article to simple geometries such as a rectangular ring. Toobtain more complex geometries requires machining the composite articleand then diffusion bonding it with other composite articles to form thefinal shape. A problem with embedding ceramic fibers in a titanium alloymatrix is that the ceramic causes the titanium to react which weakensthe alloy's bonding capability. One solution to this problem has been tocoat the fibers. However, these coatings do not totally eliminate thereaction between the alloy and the fiber and further, after machininguncoated portions of the fibers are exposed. In addition, machiningdebris and coolant residues also reduce the effectiveness and strengthof the diffusion bond.

Therefore, there is a need for a method for removing any exposed fiberor other contaminant from the surface of a machined composite article.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for removingany exposed fiber or other contaminate from the surface of a machinedcomposite article for subsequent diffusion bonding.

Another object of the present invention is to provide a method forforming metal matrix composite articles having complex shapes.

The present invention achieves this object by providing a method forpreparing a machined composite surface. The method generally involvesremoving most of the fibers by mechanical means followed by chemical andthermal cleaning to remove the remaining debris, fiber/matrix reactionzone products and coolant residues. Once the surface is machined thearticle can be diffusion bonded to adjacent articles to form complexshapes.

DESCRIPTION OF THE INVENTION

Though the following description is made in reference to a compositearticle having a metal matrix of Ti-15-3 and silicon carbide fibers, thepresent invention is applicable to all titanium alloy matrices andceramic fibers such as aluminum oxide and titanium diboride.

In the preferred embodiment, the composite article is securely mountedand then machined using diamond grinding wheels composed of a resinbonded diamond (100-320 grit at a concentration of 80-100 percent).Grinding wheels having other compositions such as silicon carbide,aluminum oxide or cubic boron nitride do not cue the very hard siliconcarbide fibers effectively and result in excessive surface damage to thearticle. A liquid coolant is sprayed on the article during machining toprevent overheating. The coolant, preferably water with a rustinhibitor, should not contain oils which can contaminate the machinedand cleaned surface during the bonding process, resulting in inadequatebond strengths.

The diamond grinding wheel should be rotated at 2000-3000 surface feetper minute. Slower speeds result in excessive grinding wheel wear andsignificantly higher speeds can overheat and damage the wheel. Duringthe finish cuts, the depth of cut is best kept at less than 0.001 inchesper pass and preferably at about 0.0002 inches to avoid damage to thefibers. The feed rate should be about 60-100 inches per minute as,higher rates cause excessive surface damage on the article.

After machining, the surfaces of the article having exposed fibers areshot peened. The shot peen must be controlled so that it breaks apartthe portion of the fibers near the surface without damaging theunderlying fiber or driving silicon carbide debris into the surfacewhere it becomes difficult to extract. To achieve this end requires theproper selection of the following parameters, shot composition and size,the delivery pressure, time, nozzle stand-off distance, and angle to thesurface being peened. The preferred parameters are steel shot at anominal diameter of 0.017 inches delivered at a pressure of 50-80 psi(the resulting Alman intensity is near 10). A smaller shot size (0.007inch diameter) tends not to break-up all the silicon carbide fiber whilelarger sizes (0.030 inch diameter or larger) created excessive damage tothe underlying fibers as well as the machined surface. This operation iseffective if performed for 8-12 minutes as the part is rotated at 8-16feet per minute. To avoid metal peen entrapment of silicon carbidedebris the nozzle should be positioned about 12 inches away from thesurface. The shot peen is easily automated to cover a complex contouredsurface by rotating the article while the shot peen nozzle is traversednormal to the machined surface.

Next, an acid etch is required to remove any remaining debris and tocondition the surface for diffusion bonding. This step includesimmersing the article in agitated methyl ethyl ketone, air drying,caustic bath immersion at about 140 F. for 5-10 minutes, water rinsing,acid immersion and agitation in an etchant solution comprising 3% HF:30%HNO₃ :Water, a thorough water rinsing, and finally air and oven dryingat 150° F. 300° F. for 1-20 hours depending upon the surface area andsize of the article.

The surface is then grit blasted to remove silicon carbide fiber debrisand residual fiber/matrix reaction zone products. Conventional gritblasting methods are employed such as the use of Silica sand deliveredat a pressure of less than 40 psi to avoid excessive erosion of themachined surface. The time required is based upon the pressure selectedbut typically would be 2-3 minutes at a pressure of 25 psi as the partis rotated at a speed of about 8-16 surface feet per minute with thenozzle located 6-12 inches from the surface, with 12 inches beingpreferred. After the grit blast the acid etch as previously described isrepeated.

The surface is then exposed, in a vacuum, to temperature in the range of900° F. to 1300° F. for 2 to 20 hours. This heat treatment removes anyremaining traces of volatile substances.

Completion of these steps results in a surface containing a minimumamount of silicon carbide fibers and where the titanium metal has beenadequately conditioned and cleaned for subsequent diffusion bonding.

Diffusion bonding is the preferred method for joining composite articleshaving matrices of a similar alloy. For titanium alloys this bondingincludes applying sufficient external pressure to cause intimate contactof the surfaces to remove voids and metal asperities. The pressure isapplied at high temperatures in a clean vacuum environment.

Vacuum hot pressing, electron beam welding or encapsulation within ametal can are all methods to provide the vacuum environment at thesurfaces to be bonded. The assembly can then be hot isostatic pressed toconsolidate and bond the machined surface to the mating structure.

Prior to hot pressing or electron beam welding, the mating surfaces, atthe periphery of the articles to be joined, are machined to a good fitand then sealed by electron beam welding, thus providing the vacuumenvironment at the mating surfaces. The electron beam welding shouldinclude an adequate vacuum pump-down time (2-16 hours) prior to makingthe final weld seal.

Alternatively, hot pressing could be used to seal the assembly whileproviding the necessary vacuum environ mete at the surfaces to bejoined. Hot pressing consists of applying a suitable pressure (2,000 to15,000 pounds per square inch) to the sealing surfaces at temperaturesappropriate to the specific titanium alloy(s) used in the assembly.These temperatures would typically be from 1300° F. to 1650° F. forconventional titanium alloys and 1600° F.-1850° F. for alpha-twotitanium aluminide type alloys. Hot pressing could also be used to bondthe sub-assemblies, given a suitable component shape, size and fiberorientation.

The mating sub-assemblies are thus readied for joining by hot isostaticpressing. Conventional HIP techniques are used which includes heatingthe articles at heating rates in the range of 500° F.-1000° F. per hour,holding at intermediate temperatures (50° F.-400° F. below the maximumtemperature), and then holding at the maximum temperature. Hold timesare selected as a function of the temperature, the higher thetemperature the less time required. Maximum temperatures range fromabout 1300° F. to 1650° F. for conventional titanium alloys and 1600° F.to 1850° F. for alpha-two titanium aluminide alloys. Pressures duringthe HIP process range from 5,000 to 30,000 pounds per square inch.

Once the bond has been completed at the HIP temperature the assembly iscooled at an appropriate rate to room temperature. The assembly is thenheat treated to develop the desired combinations of strength, fatiguelife, creep resistance and fracture toughness in the alloy matrix andadjacent structures. If there are two or more titanium alloys used,selection of that heat treatment must be accomplished in a manner thatwill enhance the performance of both alloys.

Various modifications and alterations to the above described preferredembodiment will be apparent to those skilled in the art. Accordingly,this description of the invention should be exemplary and not aslimiting the scope and spirit of the invention as set forth in thefollowing claims.

What is claimed is:
 1. A fiber-reinforced titanium or titanium alloymatrix composite article formed from a method comprising the stepsof:(a) providing a surface of said article through which a portion ofsaid fiber is exposed; (b) shot peening said surface to break apart saidexposed fiber; and (c) acid etching said surface.
 2. The article claim 1wherein said method further comprises the steps of:(d) grit blastingsaid surface to remove silicon carbide fiber debris and residualfiber/matrix reaction zone products; and (e) repeating step (c).
 3. Thearticle of claim 1 wherein step (c) of said method includes the stepsof:immersing said surface in an agitated solvent; air drying saidsurface; immersing said surface in a caustic bath at about 140° F. for5-10 minutes; rising said surface with water; immersing and agitatingsaid surface in an etchant solution; rinsing said surface with water;and air and oven drying said surface at 150° F. to 300° F. for 1 to 20hours.
 4. A composite article having a complex shape formed from amethod comprising the steps of:(a) providing at least two siliconcarbide fiber reinforced titanium or titanium alloy matrix compositearticles having simple shapes; (b) machining said articles so that whenjoined they form a complex shape, said machining exposing a portion ofsaid fiber through at least one surface of at least one of saidarticles; (c) shot peening said surface having said exposed fiber tobreak apart said fiber; (d) acid etching said surface; and (e) diffusionbonding said articles together to form a single composite article havingsaid complex shape.
 5. The article of claim 4 wherein said methodfurther comprises between steps (d) and (e) the steps of grit blastingsaid surface to remove silicon carbide fiber debris and residualfiber/matrix reaction zone products; and then repeating step (d).
 6. Thearticle of claim 5 wherein said method further comprises the step ofheat treating said surface after repeating step (d).